Northern Rockies Skies for March

February 29, 2012

A monthly look at the night skies of the northern Rocky
Mountains, written by astronomers Ron Canterna, University of Wyoming; Jay
Norris, Challis, Idaho Observatory; and Daryl Macomb, Boise State University.

For observers in the northern latitudes, the March skies
provide us with one of the most spectacular views of the Milky Way.

The Milky Way, the hazy band of unresolved stars, stretches
from the southern horizon through the constellations Gemini and Auriga; then to the she-goat, the
yellowish star Capella; passing Perseus; and then northward to the north
cardinal point on the horizon through Cassiopeia.

Orion, the prominent winter constellation you have probably
been watching most of the winter months, sets about three hours after sunset.
To the southeast of Orion, one can see Sirius, the brightest star in the sky.

Planet Heaven: Watch Venus and Jupiter (the two brightest
objects in the eastern night sky) move relative to each other during the month.
Venus gets closer to Jupiter and the closest approach (or conjunction) occurs
March 14, when they will be separated by three degrees.

Look for Mars in the night sky, with its closest approach to
the Earth on March 3. This also is called the opposition of Mars (opposite the
sun). Saturn rises around 10 p.m., so take these fantastic opportunities to
view the planets before summer arrives.

In our last discussion we considered Type Ia supernovae,
which explosions are triggered by accretion-induced collapse of a white dwarf
or coalescence of two white dwarfs. The result is catastrophic destruction of
the object and the release of an enormous amount of radiation during a period
of about a month.

As with much of astronomical nomenclature, the naming of
supernova types is arcane, and does not obviously reflect the actual causes of
the differing types of stellar explosions. Type Ib and Type Ic supernovae are similar to Type Ia's only in
that all three type I's show no evidence in their spectra for hydrogen, whereas
Type II's do exhibit hydrogen lines.

However, Type II, Type Ib and Ic are all
"core-collapse" supernovae: Single massive stars that, at the end of
their lives, consist of shells of elements. Progressively heavier elements are layered
toward the inner shells, with an iron core at the center. When fusion ceases in these objects,
core collapse ensues, triggering the supernova mechanism.

The Type Ib's and Ic's are distinguished spectrally from
Type Ia's in that no silicon lines are present in the spectra. Further, Type
Ib's show evidence of helium, but Type Ic's do not. Thus, in the progression of
core-collapse supernovae, Type II to Type Ib to Type Ic, increasingly more mass
has been blown off the star's surface by radiation (light) pressure -- or
pulled off the star by the gravitational force of a companion star -- leaving
the shells of heavier elements exposed. The Type Ib/c's are therefore referred
to as "stripped core-collapse" supernovae. Also, in this progression,
the Type Ic's have the most mass at birth, Type II's the least mass. Most
researchers who study Type Ib/c's infer
that a black hole is left behind at the site of the explosion.

This subject lies at the intersection of optical astronomers
who study the supernova light curves using ground-based telescopes; and high-energy
astrophysicists who study extreme Type Ic supernovae that are rotating very
rapidly before the explosion and, under special circumstances, may give rise to a
gamma-ray burst, to be discussed next time.